Cardiac glycosides (CG) have two well known effects on cardiac muscle, the inotropic effect and the inhibition of Na ion-K ion-ATPase. The major purpose of the present study is to correlate these two effects of CG by measuring changes of contractility and intracellular Na ion concentration simultaneously in isolated cardiac muscle following the administration of CG. During the past year's investigation it was found that the intracellular Na ion concentration was increased during the inotropic effect of cardiac glycosides. This finding will be explored further by varying the experimental conditions. Since Ca ions movement in cell components, such as cell membrane, sarcoplasmic reticulum, the modulatory proteins (troponin and tropomyosin) and possibly mitochondria, during excitation-contraction cycle is critically important in cardiac contraction and the Ca ions movement is known to be influenced by CG, the effect of CG on the interaction of Ca ions and these cell components will be investigated. For this purpose, these cellular components will be isolated from cardiac tissue and the Ca ions interaction with these components under various experimental conditions will be determined in the presence and absence of CG. Various experimental conditions to be employed are: alteration of ionic components of incubation media to induce membrane depolarization of sarcoplasmic reticulum or mitochondria, passage of electrical stimulation through incubation media containing sarcoplasmic reticulum or mitochondria, and the alteration of Ca ions concentration of media containing the modulatory proteins.